Technical Field
The present subject matter relates to a method for the wireless transmission and reception of messages in an asynchronous ad hoc network having a plurality of network nodes, at least one network node of which periodically transmits messages with a predefined period length. The invention further relates to a network node for such a method.
Background Art
Ad hoc networks for the wireless transmission and reception of messages are used, e.g., in vehicle-to-vehicle (V2V) or vehicle-to-infrastructure (V2I) communication; they are highly flexible and, due to the intermeshing of the network nodes, are failsafe. However, since every network node initiates the transmission of messages on the basis of its own internal timing, collisions occur when two or more network nodes attempt to transmit messages simultaneously. For this reason, methods for collision detection (CD) and collision avoidance (CA) are standardized for such networks having Carrier Sense Multiple Access (CSMA). These methods are inadequate, however, in the case of a large number of network nodes and/or messages, in particular: the number of successfully transmitted messages and, therefore, the efficiency of the network decreases as the number of messages increases.
Various improvements for counteracting this are known from the literature. In G. Bansal et al., “Cross-Validation of DSRC Radio Testbed and NS-2 Simulation Platform for Vehicular Safety Communications”, 2011 IEEE Vehicular Technology Conference (VTC Fall), it is proposed to reduce the transmit power of the network nodes or extend the period length of messages to be periodically transmitted in order to reduce the number of collisions. As a result thereof, the number of directly interconnected network nodes of an ad hoc network diminishes, however, and therefore messages can no longer be transmitted directly (“single hop”), but rather must be routed through a plurality of intermediate network nodes (“multi-hop”), which further increases the total number of transmitted messages; another result thereof is that the propagation speed of information is reduced, which is undesired in the case of important messages in particular, e.g., messages related to dangerous situations or accidents.
In S. Sundar et al., “Congestion Control for Vehicular Safety: Synchronous and Asynchronous MAC Algorithms”, VANET'12 Conference, Jun. 25, 2012, UK, it is proposed, in contrast, that the network nodes be synchronized, which requires a complex synchronization algorithm, which must also handle the addition or removal of individual network nodes in the entire ad hoc network, and requires a precise, common time base of one GPS receiver in every network node. It is understood that such a method functions only when all participating network nodes of the ad hoc network are gaplessly synchronized, i.e., when the ad hoc network is completely covered with such network nodes.